Download An enhanced image of the PamirHindu Kush

Geophys. J. Int. (1998) 134, 573^595
An enhanced image of the Pamir^Hindu Kush seismic zone from
relocated earthquake hypocentres
G. Pegler* and S. Das
Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK. E-mail: [email protected]
Accepted 1998 March 16. Received 1998 March 12; in original form 1997 May 28
S U M M A RY
We determine the shape of the seismic zone in the Pamir^Hindu Kush region de¢ned by
[30^420N, 68^780E] by obtaining improved hypocentral locations with 90 per cent
con¢dence limits of less than 30 km (the depth error bar for most of the earthquakes) of
about 6000 shallow and intermediate-depth earthquakes. Available S and depth-phase
arrival times are also used together with the P-wave arrival times in the joint hypocentre
determination technique. To obtain the best possible hypocentral locations, the study
region is divided into three depth ranges, 0^60, 60^160 and >160 km. The 0^60 km
depth zone is then subdivided laterally into 19 blocks, with the deeper regions divided
into two blocks each. The improved delineation of the seismic zone obtained by using
the relocated hypocentres implies that the intermediate-depth seismicity in the Pamir^
Hindu Kush region is most simply explained by a single S-shaped seismic zone, 700 km
long and no more than 30 km wide and with most activity concentrated at 100^300 km
depth. The main features observed are: (1) the eastward steepening of the north-dipping
Hindu Kush seismic zone through to its overturning at its eastern end beneath the
Pamirs, where it dips to the southeast; (2) the curvature and forking of the subducting
slab at depths greater than 200 km within the eastern part of the Hindu Kush seismic
zone; (3) the very abrupt cut-o¡ in intermediate-depth seismicity at 90^110 km depth
with no extension to shallower depths under the Pamirs, and with a persistent gap
between the intermediate and shallow seismicity in the northern Pamirs; and (4) the
unusual horizontal T-axes for intermediate-depth earthquakes of the Pamir seismic
zone, which align with its curvature. This study shows that the seismic zone under the
Hindu Kush has stress axes which follow the classical pattern for subducting slabs
controlled by gravity, whereas the Pamir region has horizontal T-axes that follow the
trend of the contorted seismic zone. This suggests that the Pamir seismic zone is a slab
deformed due to £ow in the upper mantle.
Key words: earthquakes, Pamir^Hindu Kush, P- and T-axes, subduction.
I NT ROD UC T I O N
The study of intermediate and deep earthquakes and
associated stress axes within subducting slabs was a major
piece of evidence that led to an understanding of global plate
dynamics (Isacks & Molnar 1971). Interest in intermediate and
deep intraslab earthquakes has grown recently with increasing
awareness of its importance in understanding slab metamorphism (Kirby, Engdahl & Denlinger 1996a; Kirby et al.
1996b) and its ability to provide constraints on models of
* Now at: StatSci Europe, Osney House, Mill Street, Oxford OX2 0JX,
UK.
ß 1998 RAS
mantle dynamics. Intermediate-depth (&70^300 km) earthquakes usually occur in regions where oceanic lithosphere has
been subducted for 10 Ma or longer (Chen & Molnar 1983;
Molnar 1988); intermediate-depth earthquakes not associated
with subducting oceanic lithosphere are rare. The Pamir^
Hindu Kush region of central Asia is one of the most active
regions of intermediate-depth seismicity and by far the most
active region of intermediate-depth seismicity not obviously
associated with the ongoing subduction of oceanic lithosphere. The seismicity of this region has been the subject of
many studies (e.g. Nowroozi 1971; Vinnik, Lukk & Nersesov
1977; Billington, Isacks & Barazangi 1977; Vinnik, Lukk &
Mirzokurbonov 1978; Chatelain et al. 1980; Roecker et al.
1980; Roecker 1982; Katok 1988; Ni & Fan 1989; Fan, Ni &
573
574
G. Pegler and S. Das
Wallace 1994; Mellors et al. 1995) and a variety of models have
been proposed to explain the intermediate-depth seismicity.
The models fall broadly into two categories.
(1) Models that imply that the intermediate-depth seismicity
beneath the Pamirs has a di¡erent origin from that beneath the
Hindu Kush. Such models generally propose northward subduction of the Indian plate beneath the Eurasian plate for the
Hindu Kush and southward subduction of the Eurasian plate
beneath the Pamirs (e.g. Chatelain et al. 1980; Fan & Ni 1989;
Burtman & Molnar 1993; Fan et al. 1994).
(2) Models that consider the intermediate-depth seismicity
beneath the Pamirs and Hindu Kush to be a single but highly
contorted seismic zone (e.g. Billington et al. 1977; Vinnik et al.
1977).
In this study we attempt to ascertain the con¢guration of the
intermediate-depth seismicity beneath the Pamir^Hindu Kush
region and to gain insight into the orientation of the principal
stress directions within the seismic zones by relocating the
seismicity and by examination of the Harvard centroid moment
tensor (CMT) solutions (Dziewonski et al. 1983^1994).
T E C TO N IC SET T I NG A N D PR EV I O US
S E I S M IC ST U D I E S I N T H E R E GI O N
The Pamir^Hindu Kush region is located at the western
syntaxis of the Himalayas, in the broad deformation belt
created by the collision of the Indian and Eurasian plates
(Fig. 1). The tectonic evolution of the region can broadly be
divided into three stages (Windley 1988). The ¢rst stage saw the
northward migration of several plates from Gondwanaland
and the formation of magmatic arcs, with the closure of Tethys
during the Mesozoic^Lower Tertiary. During the second stage,
the plates accreted to the southern margin of Eurasia, with the
¢nal collision, that of the Indian plate along the Indus suture
zone, occurring around 50^40 Ma. The third stage of evolution
of the region is the post-collisional indentation of the Indian
plate into Eurasia, which has resulted in as much as 2000 km of
crustal shortening across the collision zone. The intermediatedepth seismicity under study here is essentially restricted
to the Pamir^Hindu Kush region, although there are rare
intermediate-depth earthquakes beneath the Karakoram and
the Tibetan plateau. The Pamirs have an average elevation
of between 4 and 5 km and consist of a collage of sutured
terrains which accreted to Eurasia during the Triassic^Early
Cretaceous. The Karakoram lies to the south of the Pamirs and
is geographically and geologically continuous westwards into
the eastern Hindu Kush (Rex et al. 1988; Searle 1991, 1996a).
The southern boundary of the Hindu Kush and Karakoram is
marked by the Shyok suture zone, which has recently been
reactivated and is often referred to as the Main Karakoram
Thrust (MKT). The Kohistan and Ladakh blocks are island
arc terrains which lie to the south of the MKT and form
part of the 2500 km long Trans Himalayan batholith that
formed during the closure of Tethys. The present-day relative
plate motion of India with respect to Asia, within the Pamir^
Hindu Kush region, is approximately due north at 45 mm a{1
(DeMets et al. 1990). This convergence is accommodated
from the Himalayan thrust front in the south, right through
to the Tien Shan in the north (Burtman & Molnar 1993).
There is much debate concerning the present partitioning of
the total convergence across the deformation belt, in particular
the relative importance of crustal shortening by thrust
faulting or the extrusion of Eurasia along strike-slip faults
around the indenting Indian plate (Molnar & Tapponnier 1975;
Tapponnier & Molnar 1976, 1977, 1979; Burtman & Molnar
1993; Searle 1996a).
The existence of intermediate-depth earthquakes beneath
the Pamir^Hindu Kush region has long been recognized. The
¢rst published study of an earthquake in this region was the
1911 February 18 Pamir earthquake (Galitzin 1915), later also
studied by Je¡reys 1923). In a historical aside, it is interesting
to note that Turner showed that deep-focus earthquakes
existed in 1922 and Je¡reys used the 1911 Pamir earthquake,
which was a shallow shock, to argue against this possibility
in 1923! Gutenberg & Richter (1954) located several earthquakes at depths greater than 200 km beneath the Pamir^
Hindu Kush for the period 1907 to 1950 and Richter (1958)
found more than 70 earthquakes here, with nine being >7.0
magnitude at intermediate depths between 1904 and 1955.
One of the ¢rst detailed seismic sections through the Pamir^
Hindu Kush region was published by Nowroozi (1971), which
shows two thin zones of intermediate-depth seismicity. The
¢rst zone underlies the Pamirs, has a NE^SW trend, and is
de¢ned by seismicity between depths of 70 and 175 km. The
second zone beneath the Hindu Kush has an E^W trend and is
de¢ned by seismicity between 175 and 250 km depth. Vinnik
et al. (1977) suggested that the intermediate-depth seismicity
represented a single zone of activity and was caused by a rigid
region of the `tectosphere' subject to tectonic stresses from the
surrounding material. This interpretation was based on the
coincidence of a high-velocity zone at depths of up to 300 km,
the intermediate-depth seismicity beneath the Hindu Kush and
the outcrops of Precambrian rocks in the region. The ¢rst study
to suggest subduction of oceanic lithosphere as the source of
the intermediate-depth seismicity beneath the Hindu Kush and
the Pamirs was that of Billington et al. (1977). They studied
selected earthquakes from the International Seismological
Summary (ISS) and the International Seismological Center
(ISC) catalogues, as well as the best-located events of
Nowroozi (1971) and identi¢ed a contorted Benio¡ zone
running E^W beneath the Hindu Kush and NE^SW beneath
the Pamirs. They interpreted the seismicity as representing a
single seismic zone, but the possibility of there originally
having been two subduction zones of opposing polarity was
also considered. Chatelain et al. (1980) and Roecker et al.
(1980) using data from microseismicity studies in 1966, 1967,
1976 and 1977 concluded that two subducting slabs were
present and that they might represent the subduction of two
small intracontinental basins beneath the Pamirs. Much of
the interpretation of Roecker et al. (1980) and Chatelain et al.
(1980) was based on the identi¢cation of gaps in the seismicity,
which were seen in all of their data sets.
More recent geophysical studies of the Pamir region have
also favoured subduction in opposing directions beneath the
Pamir and Hindu Kush, rather than one contorted zone
(e.g. Hamburger et al. 1992; Burtman & Molnar 1993; Fan
et al. 1994). These studies conclude that continental crust
and lithosphere are subducting to depths of around 200 km
beneath the Pamirs. However, all the above-mentioned studies
that propose continental subduction beneath the Pamirs
are essentially studies concentrating on the Pamir seismic
zone alone, with little discussion of its interaction with,
or connection to, the Hindu Kush seismic zone. The local
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
575
Figure 3. Relocated seismicity for the depth range 75^100 km for the period 01/01/1964^12/31/1992, together with all available CMT
mechanisms. Solid dots represent epicentres with 90 per cent con¢dence limits ¦30 km. The size of each dot is related to the earthquake
magnitude, the size key being shown in Fig. 5. Hollow dots represent the remaining relocated events which have greater hypocentral uncertainties
and are plotted at half the diameter of the solid dots for events with equivalent mb . (Note that the smallest dots may appear solid but are actually
hollow circles.)
Figure 4. Same as Fig. 3 but for the depth range 100^125 km.
ß 1998 RAS, GJI 134, 573^595
576
G. Pegler and S. Das
Figure 5. Same as Fig. 3 but for the depth range 125^150 km. The size key relating the dot size to the earthquake magnitude is shown. The same key
is used for all seismicity plots in this paper.
studies of Roecker et al. (1980) and Chatelain et al. (1980)
generally did not consider data much beyond the limits of
the intermediate-depth seismicity. The goal of this study is to
examine a much larger seismicity data set than previously
studied. Our study area covers the entire Pamir and Hindu
Kush regions and attempts to ascertain whether the Pamir^
Hindu Kush seismic zones represent a single contorted zone of
seismicity or two unrelated seismic zones.
Figure 6. Same as Fig. 3 but for the depth range 150^175 km.
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
577
Figure 7. Same as Fig. 3 but for the depth range 175^200 km.
R E L O CAT E D S EI SM IC I T Y
To study the seismicity of the Pamir^Hindu Kush region in
detail, we have relocated about 6000 earthquakes within the
region of interest for the period 1964^1992 using phase arrival
times reported by the ISC and the joint hypocentre determination technique (JHD) developed by Dewey (1971, 1983).
The ISC reports mainly P-, some pP- and a few S-wave arrival
times. Richter (1958, Ch. 19, p. 311) wrote: `Depths are often
seriously overestimated by taking sP for pP. Sometimes a
majority of stations recording a given earthquake report a
large sP and overlook or fail to ¢nd pP. This is common with
intermediate shocks under the Hindu Kush'. The ISC has
informed us that they rarely use depth phases. We have used
available S and depth phases, in addition to the P-wave arrival
times, in this study.
In order to produce well-constrained relocated hypocentres, the region is subdivided into a number of blocks, both
horizontally and vertically. Seismicity within the Pamir^Hindu
Kush region extends to depths of around 300 km, although
at least one apparently well-constrained event has a depth
of around 380 km (Katok 1988). The ISC hypocentres appear
to be broadly divisible into three depth zones (0^60, 80^150
and 180^300 km), each zone separated by a depth band in
which there are relatively few hypocentres. Guided by this,
we divide the entire data set into three depth zones, 0^60,
60^160 and >160 km. The 0^60 km depth zone was subdivided laterally into 19 blocks. Due to the fact that the
earthquakes in the deeper regions were spatially clustered,
these two depth ranges needed to be divided only into two
blocks each. The horizontal areas of all the blocks used
are similar in size (Pegler 1995). A group of representative
ß 1998 RAS, GJI 134, 573^595
earthquakes from each block was then selected to be processed
as a joint hypocentre group. All earthquakes within each block
were then relocated using the SE89 (single-event) algorithm
(Dewey 1971, 1983) and station-phase adjustments determined
from the JHD relocation for that block. The relocated hypocentres were then combined into a single catalogue of events. If
a block did not have su¤cient well-recorded events for a JHD,
then a single-event relocation was carried out for events in that
block. The details of the relocation are fully reported in Pegler
(1995).
The ISC lists 9127 events within the region bounded by
30^420N and 68^780E for the period 1964^1992. All events for
which ISC reported fewer than six P-wave arrival times were
discarded, leaving 7151 events of which 5904 were successfully relocated. 3260 events were relocated with 90 per cent
con¢dence limits of less than 30 km. The best-constrained
earthquakes are those at intermediate depths. Of the 929
earthquakes relocated with 90 per cent con¢dence limits of
less than 10 km, 828 have depths greater than 60 km.
The relocated seismicity is shown in a series of depth ranges
and sections. To highlight the most reliably relocated events,
the events which could be relocated with 90 per cent con¢dence
limits of less than 30 km (in latitude, longitude and depth,
but usually the errors in latitude and longitude are less
than 30 km so for most events it is the error in depth) are
shown as solid dots in all seismicity plots in this paper. When
shown, the locations of events which were either not relocated
or relocated with greater uncertainty are indicated by hollow
circles and plotted at half the scale of the more reliably
relocated events. Events for which the ISC was unable to
determine depths were generally also not successfully relocated
by us.
578
G. Pegler and S. Das
Figure 8. Same as Fig. 3 but for the depth range 200^225 km.
R E L O CAT E D E A RT H QUA K E
HYPOCENTRES IN DIFFERENT
D E P T H R A NG E S A N D IN V E RT ICA L
C RO S S - S E CT I O N S
In this paper, we shall plot mainly relocated hypocentres, the
comparison with ISC hypocentres for all cases being shown
in Pegler (1995). In general, we ¢nd that after relocation
the hypocentres lie in narrower zones than those de¢ned by the
ISC estimates.
The relocated seismicity, in di¡erent depth ranges, is shown
in Figs 2^9. It is also shown in a series of twelve 800 km long
sections, the locations of these sections being shown in Fig. 10.
The six sections N^N', O^O', P^P', Q^Q', R^R' and S^S'
shown in Figs 11^16 are 50 km wide N^S sections roughly
perpendicular to the Hindu Kush seismic zone. The four
sections T^T', U^U', V^V' and W^W' shown in Figs 17^20
are 50 km wide sections taken perpendicular to the dip of the
Pamir seismic zone. A 120 km wide section, X^X', across the
eastern end of the Pamir seismic zone is shown in Fig. 21, and
a 120 km wide section, Y^Y', parallel to and encompassing
almost all of the intermediate-depth seismicity of the Pamir
seismic zone is shown in Fig. 22. In order to examine the region
where the Hindu Kush and Pamir seismic zones `meet', a set
of thinner, more arc-perpendicular sections, Z1 to Z8, are
presented. Fig. 23 shows the location of this set of sections,
the sections being shown in Figs 24 and 25. Fig. 26 shows the
seismicity of the Karakoram region in map view and in a
section denoted KA^KA'.
A complete description of the relocated seismicity in all
depth ranges and in all vertical sections is given in Pegler
(1995). We discuss only the most important features in the
Appendix and summarize the results in the next section.
SU M M A RY OF T H E FE AT U R E S S E E N I N
T HE R E L O CAT E D SE I SM IC I T Y
The intermediate-depth seismicity beneath the Pamir^Hindu
Kush region occurs in a 700 km long S-shaped zone, no wider
than 30 km throughout most of its length and with most
activity concentrated between depths of 100 and 300 km. This
zone is not continuous throughout its length, nor at all depths
within any given section, but includes highly active clusters
of events and several seismic gaps. The seismic zone dips
towards the north beneath the Hindu Kush and to the southeast beneath the Pamirs. Seismicity along the Hindu Kush
seismic zone suggests strongly that the zone extends to
depths as shallow as 60 km and may extrapolate to the surface
in the region of the MMT or MKT. Beneath the Hindu
Kush, the northward dip of the seismic zone, at depths greater
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
579
Figure 9. Same as Fig. 3 but for the depth range 225^250 km. Error ellipses represent projections of the 90 per cent con¢dence ellipsoid.
than 100 km, steepens from around 500 at 690E, becomes
vertical and overturns at its eastern end, at around 71.50E. As
the Hindu Kush seismic zone becomes vertical, seismicity
indicating N^S shortening occurs just to the north of and
slightly shallower than the Hindu Kush seismic zone. Almost
all CMT mechanisms within the Hindu Kush seismic zone
between 100 and 200 km depth have subvertical T-axes, and
B-axes which closely approximate the arcuate strike of the
seismic zone. There is an inverted V-shaped fork in the Hindu
Kush seismic zone at its eastern end, at depths greater than
200 km.
Intermediate-depth seismicity of the Pamir seismic zone
displays less activity than the Hindu Kush seismic zone and
appears to end abruptly at between 90 and 110 km depth and
nowhere does it clearly extend to shallower depths. Seismicity
along the northern margin of the Pamirs extends to depths
of 70^80 km in some sections, but the minimum horizontal
distance between this seismicity and that of the intermediatedepth seismic zone beneath the Pamirs is 70 km. CMT mechanisms along the Pamir seismic zone are more varied than
along the Hindu Kush seismic zone. Unusual horizontal T-axes
paralleling the strike of the seismic zone suggest that bending
along the seismic zone is the dominant feature east of 720E.
Thus, the stresses in the slab here do not have the classical
pattern for downgoing slabs controlled by gravity but appear
to depend instead on the contortion of the seismic zone.
ß 1998 RAS, GJI 134, 573^595
Finally, seismicity beneath the Karakoram appears to de¢ne a
triangular wedge down to 100 km, which is bounded at the
surface by the MKT in the southwest and by the southern
margin of the Tarim basin in the northeast.
D I S C USS I O N A N D C O NC LUS I O N S
Whereas most studies conclude that the intermediate-depth
seismicity beneath the Pamir^Hindu Kush region represents
some form of subduction of lithosphere, there has been much
debate in the literature as to the nature of the material that
the seismicity represents and whether it is caused by one or
two opposing subduction zones (e.g. Billington et al. 1977;
Chatelain et al. 1980; Hamburger et al. 1992; Burtman &
Molnar 1993; Fan et al. 1994). We discuss the main factors
surrounding this debate with emphasis on the implications of
the evidence from this and other studies of the seismicity of the
region. We shall argue that the Pamir^Hindu Kush seismic
zone displays several features which are more simply and
elegantly explained by a single contorted zone of seismicity.
What does the intermediate-depth seismicity represent?
The results of this and other studies of the Pamir^Hindu Kush
seismic zone indicate that the intermediate-depth seismicity
occurs along thin (<30 km wide) slab-like seismic zones
580
G. Pegler and S. Das
Figure 11. Relocated seismicity section N^N'. Hypocentres plotted are for the period 01/01/1964^12/31/1992. Solid dots represent hypocentres
with 90 per cent con¢dence limits ¦30 km. Hollow dots represent the remaining relocated events which have greater hypocentral uncertainties and
are plotted at half the diameter of the solid dots for events with equivalent mb . (Note that the smallest dots may appear solid but are actually hollow
circles.) CMT mechanisms are plotted as back-hemisphere projections with compressional quadrants shaded. The numbers at the centre of the top
edge of the ¢gure give the longitude (E) and latitude (N) of the centre of the section, and the abscissa the distance in kilometres from this point. The
topographic pro¢le (height in metres) along the line of the section is plotted above the seismicity section.
(e.g. Billington et al. 1997; Roecker et al. 1980; Chatelain et al.
1980). These zones are similar to those seen along present-day
subduction zones where oceanic lithosphere is being subducted. The near-vertical T-axes seen along the Hindu Kush
seismic zone at intermediate depths are commonly seen within
subducting oceanic plates which descend to intermediate depths
(Isacks & Molnar 1971; Billington et al. 1977; Roecker et al.
1980; Chatelain et al. 1980). Therefore, the intermediate-depth
seismicity beneath the Pamir^Hindu Kush might be considered
to represent subducted oceanic crust (e.g. Billington et al. 1977;
Chatelain et al. 1980). However, the youngest oceanic rocks
within the western syntaxis of the Himalayas represent the
closure of Tethys some 50^40 Ma, and furthermore there is
no evidence for any subduction-related volcanism within the
Figure 12. Same as Fig. 11 but for the seismicity section O^O'.
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
581
Figure 13. Seismicity section P^P'.
Tertiary. Assuming the Hindu Kush seismic zone reaches the
surface in the vicinity of the MMT at 690E, the total subducted
along-slab length of the Hindu Kush seismic zone, down to
300 km depth, is around 500 km. The present-day convergence
rate between India and Eurasia is about 45 mm a{1 (DeMets
et al. 1990). At this rate it would take about 10 Ma to subduct
a 500 km long piece of lithosphere. Even at only half this
rate it would still only take around 20 Ma to subduct 500 km
of lithosphere. Therefore, it is very unlikely that any of the
seismicity at depth beneath the Hindu Kush can represent any
Figure 14. Seismicity section Q^Q'.
ß 1998 RAS, GJI 134, 573^595
part of the former Tethys oceanic lithosphere; it must represent
more recent subduction. Chatelain et al. (1980) argued that the
lack of recent volcanism within the region, combined with
the presence of intermediate-depth seismicity, indicates a very
brief period of subduction in the late Tertiary, involving
the subduction of two small oceanic basins beneath the Hindu
Kush and Pamirs. However, as mentioned above, there is
absolutely no geological evidence for the existence of any such
basins. Other authors have suggested that the intermediatedepth seismicity represents subduction of continental crust
582
G. Pegler and S. Das
Figure 15. Seismicity section R^R'. The inset shows seismicity for the same orientation and scale as the main ¢gure, but for a section with its centre
displaced by half a degree in latitude and extending from 200 to 400 km. The main purpose of this inset is to show the seismicity below 300 km,
especially one earthquake near 400 km depth.
(e.g. Roecker 1982; Hamburger et al. 1992; Burtman & Molnar
1993; Fan et al. 1994). Roecker (1982) found evidence for
low velocities associated with the Hindu Kush seismic zone,
shallower than 200 km, from a local tomographic inversion
for P and S velocities, and suggested that this was indicative of
the subduction of continental crust, rather than oceanic crust.
Vinnik et al. (1977), however, found evidence for abnormally
high velocities at greater than 200 km depth within the Hindu
Kush seismic zone. A more recent study by Mellors et al.
(1995) also implies the presence of a high-velocity zone at
greater than 200 km beneath the Hindu Kush seismic zone.
Results presented in Pegler (1995) con¢rm the presence of
a high-velocity zone beneath the Hindu Kush, but do not
resolve at what depth it occurs. Several authors point out
that the existence of a low-velocity zone shallower than 200 km
and a high-velocity zone deeper than 200 km depth beneath the
Hindu Kush need not be incompatible and might represent
the transition between the subduction of continental and
Figure 16. Seismicity section S^S'.
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
583
Figure 17. Seismicity section T^T'.
oceanic crust (e.g. Roecker 1982; Burtman & Molnar 1993;
Mellors et al. 1995). Beneath the Pamirs, there is little clear
evidence for the existence of either unusually high or unusually
low velocities. It has been suggested that the Pamir seismic
zone represents the subduction of Asian continental crust
down to depths of around 200 km (Hamburger et al. 1992;
Burtman & Molnar 1993; Fan et al. 1994). Whether or not
continental crust can be subducted to depths of 200 km is
unclear (Burtman & Molnar 1993) and no examples of continental subduction to such depths within other regions have
been convincingly demonstrated.
Figure 18. Seismicity section U^U'.
ß 1998 RAS, GJI 134, 573^595
Continuity of the Pamir^Hindu Kush seismic zone
To ascertain whether the Pamir and Hindu Kush seismic zones
represent one or two subducting plates, it is important to
examine the region where the two zones approach each other
most closely. Billington et al. (1977) ¢rst described the Pamir^
Hindu Kush seismic zone as a single contorted zone, in which
part of the zone beneath the Pamirs may have become overturned. However, Roecker et al. (1980) and Chatelain et al.
(1980) using data recorded over relatively short periods from
local stations showed the existence of a 50 km gap between the
584
G. Pegler and S. Das
Figure 19. Seismicity section V^V'.
Pamir and Hindu Kush seismic zones. Roecker et al. (1980) and
Chatelain et al. (1980) cited this gap as the most convincing
evidence that the two zones represented two subducting
slabs, rather than a single subducting slab that had overturned beneath the Pamirs. The data from this study also
shows a gap of around 50 km, at all depths, between the
Pamir and Hindu Kush seismic zones, but we do not consider
this as strongly suggesting that the two seismic zones are
therefore representative of two di¡erent directions of subduction. First, this gap is not the only gap within the 700 km
long intermediate-depth seismic zone. A smaller, 30 km gap
exists around 750E within the Pamir seismic zone, and within
any 25 km depth range there are numerous 20^50 km gaps
along the seismic zone. The gap between the Pamir and Hindu
Kush seismic zones would appear to be the largest but its
existence need not favour interpretation of the Pamir^Hindu
Kush zone as two separate subduction zones, rather than one
contorted subduction zone. In the one-slab model, the gap in
seismicity could be interpreted as evidence for a tear in the
plate where it has overturned and detached from its shallower
continuation. Assuming the intermediate-depth seismicity
occurs along a particular interface (or narrow zone), it is then
possible that, owing to the tear, there is a length over which the
interface no longer exists, resulting in a gap in seismicity. In
the two-slab model, the gap occurs between two plates with
opposing subduction directions. We are not aware of any other
Figure 20. Seismicity section W^W'.
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
585
Figure 21. Seismicity section X^X'.
region where two subducting slabs, subducting in opposing
directions, are geographically as close to each other as the twoslab model for the Pamir^Hindu Kush region would require.
Perhaps the best example of two opposing subduction zones in
close proximity is between the Manila and Philippine trenches
beneath the Philippine Islands. However, the gap between
earthquakes associated with the two subducting slabs at depths
>150 km beneath the Philippines is over 300 km. For depths
below 100 km, the extrapolation of the trends of the Pamir and
Hindu Kush seismic zones across the seismic gap between them
shows a remarkable alignment of the zones. The Hindu Kush
seismic zone bends signi¢cantly from an E^W trend in the west
to a more NE^SW trend in the east, resulting in an apparent
alignment with the strike of the Pamir seismic zone. If the two
zones are of separate origin, one should expect to see a di¡erence in their trends, or an o¡set between the depth contours
either side of the gap that separates the two zones, as shown by
Leith & Simpson (1986), but this is certainly not the case.
Figure 22. Seismicity section Y^Y'.
ß 1998 RAS, GJI 134, 573^595
The inferred bend in the Hindu Kush seismic zone at its
eastern end leads to a problem of spatial accommodation of
the bent slab at depth. This problem would be ameliorated if
the fault identi¢ed by Billington et al. (1977) within the northdipping Hindu Kush seismic zone extends further to the east
than suggested by Billington et al. (1977), or, alternatively, if
a separate fault exists within the eastern part of the seismic
zone at depths greater than 200 km. This inferred fault not
only extends across the near-vertical Hindu Kush seismic zone
but also extends beyond the zone and forms the inverted V
displayed most clearly in section Z3 (Fig. 24). This forked
feature is also seen in the data of Roecker et al. (1980), but
not in the data of Chatelain et al. (1980). This indicates that
although their local data may provide accurate hypocentres,
the brief periods of operation of the networks do not give a
complete picture of the geometry of the Pamir^Hindu Kush
seismic zone. Indeed, the gap in seismicity around 70.60E
at 200^250 km depth seen within all of the local studies of
586
G. Pegler and S. Das
Figure 23. Location map of Z seismicity sections. Epicentres plotted are for the period 01/01/1964^12/31/1992 for the depth range 75^300 km.
Only epicentres with 90 per cent con¢dence limits ¦30 km are shown.
Roecker et al. (1980) and Chatelain et al. (1980) is ¢lled
by both the ISC and the relocated events presented in this
study.
Surface expressions of the intermediate-depth
seismicity
In considering whether the Pamir^Hindu Kush seismicity
represents one or two directions of subduction, it is important
to ascertain if the intermediate-depth seismicity continues to
shallower depths, or if it can be associated with any sutures or
fault systems at the surface. As clearly shown in the seismic
sections within this study, the Hindu Kush seismic zone
extends as shallow as 60 km and possibly reaches the surface in
the region of the MMT or MKT. Indeed, it is widely accepted
that the Hindu Kush seismic zone represents northward subduction (Billington et al. 1977; Roecker et al. 1980; Chatelain
et al. 1980; Hamburger et al. 1992; Burtman & Molnar 1993).
However, as shown above, it is less clear whether the Pamir
seismic zone represents the subduction of Asian lithosphere to
depths of 200 km beneath the Pamirs. Studies which adhere
to subduction of the Asian lithosphere to the southeast beneath
the Pamirs either point to the Tien Shan (Hamburger et al.
1992) or the northern margin of the Pamirs (Chatelain et al.
1980; Burtman & Molnar 1993; Fan et al. 1994) as the surface
locus of the subduction (Fig. 27a). The CMT mechanisms of
Fig. 2 clearly demonstrate signi¢cant N^S shortening beneath
the Tien Shan and northern Pamirs, although strike-slip
faulting also contributes signi¢cantly to deformation along
the northern margin of the Pamirs. Hamburger et al. (1992)
studied the shallow seismicity beneath the Tien Shan and
northern Pamirs, and presented a single NW^SE seismicity
section which most closely corresponds to section W^W'
(Fig. 20). Hamburger et al. (1992) interpreted their seismicity
section as thrusting beneath the Peter I range within the
0^12 km depth range, underlain by thrusting in the 17^35 km
depth range, which extrapolates to the surface in the Tien
Shan. Hamburger et al. (1992) further postulated that this
south-dipping zone might be an up-dip continuation of the
intermediate-depth seismicity of the Pamir seismic zone. The
shallow seismicity beneath the northern Pamirs and Tien
Shan in section W^W' shows similar trends to the section of
Hamburger et al. (1992), with a zone of shallow seismicity
extending from the surface at the southern margin of the South
Tien Shan and dipping southwards to about 40^50 km depth.
However, if this zone does link up with the Pamir seismic zone
at depth, then it is aseismic over a length of 140 km, between
about 40 and 100 km depth. Invariably, published sections
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
587
Figure 24. Seismicity sections Z1^Z4. Hypocentres plotted are for the period 01/01/1964^12/31/1992. Only hypocentres with 90 per cent
con¢dence limits ¦30 km are shown. Open boxes represent the projection of CMT T-axes onto the plane of each section. Each of the sections is
20 km wide and only contains hypocentres with 90 per cent con¢dence limits of less than 30 km, as well as the projections of all available T-axes
obtained by the CMT solution.
across the Pamir seismic zone show a clear gap between
the intermediate-depth seismicity and the shallower seismicity
along the northern margin of the Pamirs. The gap between the
intermediate-depth seismicity of the Pamir seismic zone and
the shallow seismicity beneath the northern Pamirs and Tien
Shan is shortest in seismic sections T^T' and U^U' (Figs 18
and 19), where it reaches a minimum of 50^60 km. Burtman &
Molnar (1993) identify an aseismic portion of the subducting
Asian lithospheric plate, between depths of about 40 and
100 km. However, sections T^T' and U^U' imply that the gap
in seismicity between the two zones of activity corresponds
to a much smaller depth range, and is more of a horizontal
gap, rather than an aseismic depth window. This con¢guration
is more consistent with two seismic zones of separate origin
than with subduction of Asian lithosphere down to 200 km
depth. Nowhere does the intermediate-depth Pamir seismic
zone extend shallower than 90 km, and its top appears to be
marked by an abrupt cut-o¡ in activity. Furthermore, T-axes
within a substantial part of the Pamir seismic zone indicate
ß 1998 RAS, GJI 134, 573^595
that along-arc bending is the most signi¢cant force acting
within the seismic zone. This stress pattern would not be
expected within an actively subducting slab, continuous from
the surface to a depth of around 200 km. All of the above
observations suggest that the intermediate-depth Pamir
seismic zone is not due to subduction of Asian lithosphere
down to 200 km depth. N^S shortening is clearly occurring
within the northern Pamirs and Tien Shan, but there is no
conclusive evidence to suggest that this extends to depths
greater than 50 km and certainly not to 200 km. The topography of the region highlights another di¡erence between the
Pamir and Hindu Kush seismic zones. Sections N^N', O^O'
and P^P' show a marked topographic high (the Hindu Kush)
above the region where the Hindu Kush seismic zone bends
sharply at around 100 km depth. The topographic high is
greatest above the centre of the Hindu Kush seismic zone
(sections O^O' and P^P') and decreases to the east and west. In
section R^R', where the seismic zone in the one-plate model is
interpreted to tear as it overturns, the topography shows a
588
G. Pegler and S. Das
Figure 25. Same as Fig. 24 but for the seismicity sections Z5^Z8.
more gentle incline northwards into the Pamirs. Beneath the
Pamir seismic zone, the topography is essentially £at, above
what would be the bend in the subducting Asian lithosphere,
in the two-subducting-plates model. We do not suggest that
there is a causal link between the topographic relief and the
geometry of the seismic zones at depth, but merely point out
the di¡erence in topography above the two seismic zones.
Steepening and along-strike termination of the Hindu
Kush seismic zone
Another feature of the Pamir^Hindu Kush seismic zone
which is more easily explained by a single seismic zone, rather
than two opposing seismic zones, is the steepening of the
Hindu Kush seismic zone from west to east, becoming vertical
and then overturning at its eastern end. This steepening
was noted by Chatelain et al. (1980), who suggested that it
might be caused by the indenting Indian plate. However,
Chatelain et al. (1980) preferred not to extrapolate this model
through to the ultimate tearing and overturning of the eastern
part of the seismic zone. It seems unnecessary to interpret the
Pamir seismic zone, which dips steeply to the southeast and
displays seismicity within the same depth range as the Hindu
Kush seismic zone, only 50 km away, as a separate feature,
especially as the Hindu Kush seismic zone itself overturns to
dip to the southeast at its eastern end.
To the west, the Hindu Kush seismic zone is clearly bounded
by the sinistral strike-slip Darvaz and Chaman fault systems.
The Chaman fault system extends for around 1000 km to the
south of the Hindu Kush and marks the western boundary of
the indenting Indian plate. The eastern Hindu Kush appears to
continue geologically eastwards into the Karakoram (Searle
1996a) with no major suture or fault system separating the
two mountain ranges. One question requiring an explanation
is why the high level of seismicity beneath the Hindu Kush
is not continued east of 71.60E, beneath the Karakoram. It is
suggested here that the seismicity now underlying the Pamirs
was formerly an eastward continuation of the seismicity
beneath the Hindu Kush, and represents what was once a
700 km long, north-dipping subduction zone that underlay the
Hindu Kush and Karakoram.
The eastern limit of the intermediate-depth seismicity of
the Pamir seismic zone is approximately along the northern
extension of the Karakoram fault up into the northeastern
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
589
Seismicity under the Karakoram
(a)
(b)
(c)
Figure 27. (a) Schematic section through the Pamir region (Fan et al.
1994, copyright by the American Geophysical Union). GKF~Gissal^
Kokshal fault; PT~Pamir Thrust; MBT~Main Boundary Thrust;
MMT~Main Mantle Thrust; IS~ Indus^Tsangpo Suture. (b) Our
interpretation of the Pamir^Hindu Kush seismic zone. HK~Hindu
Kush seismic zone; P~Pamir seismic zone; KaF~Karakoram Fault,
N~north. The Pamir seismic zone is concluded to have torn away from
the Hindu Kush seismic zone and overturned at depths greater than
90 km. The stippling and the two dotted lines with arrows are used to
illustrate this over-turning. (c) Schematic section for the Karakoram
from Fan et al. (1994, copyright by the American Geophysical Union).
MCT~Main Central Thrust; MBT~Main Boundary Thrust;
MFT~Main Frontal Thrust.
margin of the Pamirs. It is further suggested that the
Karakoram fault marks the eastern limit of the Pamir^
Hindu Kush seismic zone, and that dextral movement along
the Karakoram fault has accommodated the overturning of
the single, north-dipping, subducting slab to the east of 71.60E.
Fig. 27(b) shows a possible con¢guration of the Pamir^Hindu
Kush seismic zone at depth, based on the model of a single,
northward-subducting system, which has torn and overturned
beneath the Pamirs.
ß 1998 RAS, GJI 134, 573^595
Fan et al. (1994) suggested that the subduction of the Asian
lithosphere beneath the Pamirs extends further to the east
beneath the Karakoram. Fan et al. (1994) interpreted their
section D^D' (Fig. 16a of Fan et al. 1994) as representing steep
subduction of the Asian plate, which is being bent down by
the more gently dipping Indian plate (Fig. 27c). However,
section D^D' of Fan et al. (1994) is over 200 km wide and the
seismicity at greater than 100 km depth is seismicity projected
from the Pamir seismic zone. We have relocated the seismicity
in the Karakoram region, shown in Fig. 26 in map view and
section. Section KA^KA' reveals a much clearer picture of the
Karakoram seismicity than section D^D' of Fan et al. (1994).
Note that the CMT mechanism plotted in Fig. 26, outside
section KA^KA', is event 10 of Fan et al. (1994), which was
included within section D^D' of Fan et al. (1994), and indicates
how intermediate-depth seismicity from the Pamir seismic zone
was included within their seismicity section for the Karakoram.
Section KA^KA' de¢nes a triangular wedge, bounded by the
MKT to the southwest and the Tarim basin to the northeast, producing a pop-up structure for the Karakoram. This
interpretation ¢ts in well with a geological section taken across
the Karakoram (Searle & Tirrul 1991) in which the MKT is
interpreted as a deep-seated, breakback thrust, extending to
around 100 km beneath the Karakoram. Section KA^KA' also
implies thrusting of the Kunlun over the Tarim, by as much as
200 km and down to 100 km depth. A gravity pro¢le across the
Karakoram, Kunlun and Tarim basin would appear to support
the above ¢ndings.
Fig. 28 shows the isostatic gravity anomaly pro¢le across
the region (Molnar 1988). Three negative anomalies are seen
which correspond to the foreland basin of the Himalayas, the
Karakoram and the Tarim basin (Molnar 1988). Molnar (1988)
suggested that in order to account for the gravity anomaly
across the southern margin of the Tarim basin by a simple plate
model, in which the Tarim basin underthrusts the Kunlun, the
thrusting would probably have to extend over 100 km southwest beneath the Kunlun. Such a length of underthrusting is
consistent with the seismicity of section KA^KA'. Molnar
(1988) went on to argue that the negative gravity anomaly
beneath the Karakoram was evidence for abnormally cold
mantle, which is consistent with what might be expected
beneath a zone of intense crustal shortening. Furthermore,
Molnar (1988) suggested that the downwelling of mantle
material beneath the Karakoram may be suppressing the
Moho in this region, allowing the unusually deep seismicity
(Fig. 28). In the two-slab model for the Pamir^Hindu Kush
seismic zone, the Karakoram region lies well to the south of
the area where downwelling of the mantle might be expected.
However, within the single-slab model the Karakoram marks
the region where most downwelling of mantle material is
expected.
Implications for mantle dynamics
The seismic zone under the Hindu Kush has stress axes which
follow the classical pattern for subducting slabs controlled by
gravity. The Pamir region, however, has horizontal T-axes
that follow the trend of the contorted seismic zone. This
implies that the Pamir seismic zone is under horizontal shear
and appears to be a slab caught up in £ow within the upper
590
G. Pegler and S. Das
acceleration (mGal)
ISOSTATIC
GRAVITY
ANOMALIES
100
0
–100
elevation (m)
KARAKORAM
HIMALAYA
KUNLUN
5000
TARIM BASIN
INDUS
PLAIN
0
Crust
Mantle
Lithosphere
Asthenosphere
500 km
Figure 28. Sketches of isostatic gravity anomalies, topography and
an interpreted section across the Himalaya, Karakoram and Kunlun
(after Molnar 1988).
mantle. The observed seismicity under the Pamirs is thus a
manifestation of the deformation of the slab induced by this
£ow.
AC K NOW L E D GM E N T S
One of the authors (GP) was supported by the NERC
studentship GT4/91/GS/109. We would like to thank Jim
Dewey for use of his earthquake relocation programs. Gary
Pavlis provided profound thought-provoking comments and
we are very grateful to him for this.
R E F E R E NCE S
Avouac, J.-P. & Tapponnier, P., 1993. Kinematic model of active
deformation in central Asia, Geophys. Res. Lett., 20, 895^898.
Billington, S., Isacks, L.B. & Barazangi, M., 1977. Spatial distribution
and focal mechanisms of mantle earthquakes in the Hindu Kushö
Pamir region; a contorted Benio¡ zone, Geology, 5, 699^704.
Burtman, V.S. & Molnar, P., 1993. Geological and geophysical
evidence for deep subduction of continental crust beneath the
Pamir, Tech. rept. Spec. Pap., 281, Geological Society of America.
Chatelain, J.L., Roecker, S.W., Hatzfeld, D. & Molnar, P., 1980.
Microearthquake seismicity and fault plane solutions in the Hindu
Kush region and their tectonic implications, J. geophys. Res., 85,
1365^1387.
Chen, W.P. & Molnar, P., 1983. Focal depths of intracontinental and
intraplate earthquakes and their implications for the thermal
and mechanical properties of the lithosphere, J. geophys. Res., 88,
4183^4214.
DeMets, C., Gordan, R.G., Argus, D.F. & Stein, S., 1990. Current
plate motions, Geophys. J. Int., 101, 425^478.
Dewey, J.W., 1971. Seismic studies with the method of joint hypocenter determination, PhD thesis, University of California,
Berkeley, CA.
Dewey, J.W., 1983. Relocation of instrumentally recorded pre-1974
earthquakes in the South Carolina region, in Studies Related to the
Charleston, South Carolina, Earthquake of 1886öTectonics and
Seismicity, pp. Q1^Q9, ed. Gohn, G.S., US Geol. Surv. Prof.
Paper 1313.
Dziewonski et al. 1983^1994. Centroid-moment tensor solutions
1977^1993, Phys. Earth planet. Inter., 33^83.
England, P.C. & Houseman, G.A., 1989. Extension during continental
convergence with applications to the Tibetan Plateau, J. geophys.
Res., 94, 17 561^17 579.
Fan, G. & Ni, J.F., 1989. Source parameters of the 13 February 1980,
Karakoram earthquake, Bull. seism. Soc. Am., 79, 945^954.
Fan, G., Ni, J.F. & Wallace, T.C., 1994. Active tectonics of the Pamirs
and Karakoram, J. geophys. Res., 99, 7131^7160.
Galitzin, B., 1915. Sur le tremblement de terre du 18 fevrier 1911,
Comptes Rendues, 160, 810^813.
Gutenberg, B. & Richter, C.F., 1954. Seismicity of the Earth and
Associated Phenomena, Princeton University Press, Princeton,
NJ.
Hamburger, M.W., Sarewitz, D.R., Pavlis, T.L. & Popandopulo, G.A.,
1992. Structural and seismic evidence for intracontinental subduction in the Peter the First range, central Asia, Geol. Soc. Am.
Bull., 104, 397^408.
Isacks, B. & Molnar, P., 1971. Distribution of stresses in the descending
lithosphere from a global survey of focal mechanism solutions of
mantle earthquakes, Rev. Geophys. Space Phys., 9, 103^174.
Je¡reys, H., 1923. The Pamir earthquake of 1911 February 18, in
relation to the depths of earthquake foci, Month. Not. R. astr. Soc.,
Geophys. Suppl., 1, 22^31.
Katok, A.P., 1988. On the deepest earthquake in the PamiröHindu
Kush zone, Izv. Acad. Sci. USSR Phys. Solid Earth, Transl., 24,
649^653.
Kirby, S., Engdahl, E.R. & Denlinger., 1996a. Intermediate-depth
intraslab earthquakes and arc volcanism as physical expressions of
crustal and uppermost mantle metamorphism in subducting slabs,
in Subduction: Top to Bottom, American Geophysical Union,
Geophys. Monogr., Vol. 96, Washington, DC.
Kirby, S.H., Stein, S., Okal, E.A. & Rubie, D.C., 1996b. Metastable
mantle phase transformations and deep earthquakes in subducting
oceanic lithosphere, Rev. Geophys., 34, 261^306.
Leith, W. & Simpson, D.W., 1986. Seismic domains within the
Gissar-Kokshal seismic zone, Soviet central Asia, J. geophys. Res.,
91, 689^697.
Mellors, R.J., Pavlis, G.L., Hamburger, M.W., Al-Shukri, H.J. &
Lukk, A.A., 1995. Evidence for a high-velocity slab associated with
the Hindu Kush seismic zone, J. geophys. Res., 100, 4067^4078.
Molnar, P., 1988. A review of geophysical constraints on the deep
structure of the Tibetan Plateau, the Himalaya and the Karakoram
and their tectonic implications, Phil. Trans. R. Soc. Lond., A326,
33^88.
Molnar, P. & Tapponnier, P., 1975. Cenozoic tectonics of Asia, e¡ects
of a continental collision, Science, 189, 419^426.
Ni, J. & Fan, G.W., 1989. Fault plane solutions of earthquakes and
active tectonics of the Pamir-Karakoram region, EOS, Trans. Am.
geophys. Un., 70, 1226.
Nowroozi, A.A., 1971. Seismotectonics of the Pakistan plateau,
eastern Turkey, Caucasus, and Hindu Kush regions, Bull. seism.
Soc. Am., 61, 317^341.
Pavlis, G.L. & Hamburger, M.W., 1991. Aftershock sequences of
intermediate-deep earthquakes in the Pamir-Hindu Kush seismic
zone, J. geophys. Res., 96, 18 107^18 117.
Pegler, G., 1995. Studies in seismotectonics, PhD thesis, Department of
Earth Sciences, University of Oxford.
Rex, A.J., Searle, M.P., Tirrul, R., Crawford, M.B., Prior, D.J.,
Rex, D.C. & Barnicoat, A., 1988. The geochemical and tectonic
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
evolution of the central Karakoram, North Pakistan, Phil. Trans. R.
Soc. Lond., A326, 229^255.
Richter, C.F., 1958. Elementary Seismology, W.H. Freeman & Co.,
San Francisco, CA.
Roecker, S.W., 1982. Velocity structure of the Pamir-Hindu Kush
region: possible evidence of subducted crust, J. geophys. Res., 87,
945^959.
Roecker, S.W., Soboleva, V., Nersesov, I.L., Lukk, A.A., Hatzfeld, D.,
Chatelain, J.L. & Molnar, P., 1980. Seismicity and fault plane
solutions of intermediate depth earthquakes in the Pamir-Hindu
Kush region, J. geophys. Res., 85, 1358^1364.
Searle, M.P., 1991. Geology and Tectonics of the Karakoram Mountains,
John Wiley, New York, NY.
Searle, M.P., 1996a. Geological evidence against large-scale preHolocene o¡sets along the Karakoram fault: implications for the
limited extrusion of the Tibetan plateau, Tectonics, 15, 171^186.
Searle, M.P., 1996b. Geological Map of North Pakistan (and adjacent
areas of northern Ladakh and western Tibet) 1 : 650,000 scale, Dept
Earth Sciences, Oxford University.
Searle, M.P. & Tirrul, R., 1991. Structure and thermal evolution of the
Karakoram crust, J. geol. Soc. Lond., 148, 65^82.
Tapponnier, P. & Molnar, P., 1976. Slip-line ¢eld theory and large scale
continental tectonics, Nature, 264, 319^324.
Tapponnier, P. & Molnar, P., 1977. Active faulting and tectonics in
China, J. geophys. Res., 82, 2905^2930.
Tapponnier, P. & Molnar, P., 1979. Active faulting and Cenozoic
tectonics of the Tien Shan, Mongolia, and Baykal regions,
J. geophys. Res., 84, 3425^3459.
Turner, H.H., 1922. On the arrival of earthquake waves at the antipodes, and the measurement of the focal depth of an earthquake,
Mon. Not. Ry. Astr. Soc. Geophys. Suppl., 1, 1^13.
Vinnik, L.P., Lukk, A.A. & Nersesov, I.L., 1977. Nature of the intermediate seismic zone in the mantle of the Pamir-Hindu Kush,
Tectonophysics, 38, T9^T14.
Vinnik, L.P., Lukk, A.A. & Mirzokurbonov, M., 1978. Quantitative
analysis of velocity inhomogeneities of the PamirsöHindu Kush
upper mantle, Izv. Acad. Sci. USSR Phys. Solid Earth, Transl., 14,
319^328.
Windley, B.F., 1988. Tectonic framework of the Himalaya, Karakoram
and Tibet, and problems of their evolution, Phil. Trans. R. Soc.
Lond., A326, 3^16.
A PPE N D I X A : D ETA I LE D D I S C US S I O N OF
R E L O CAT E D S EI SM IC I T Y
Relocated seismicity in di¡erent depth ranges
1. 0^75 km depth range (Fig. 2)
Most of the shallow activity is concentrated along the narrow
band extending from 690E, 38.50N to 780E, 400N, along the
northern margin of the Pamirs. This is the Gissar^Kokshal
seismic zone (Leith & Simpson 1986), de¢ned by the Darvaz^
Karakul and Vakhsh fault systems. The Darvaz fault marks the
boundary between the northwestern margin of the Pamirs and
the Tadjik basin, and trends roughly NE^SW, merging with the
E-W-trending Vakhsh thrust around 710E, 39.50N. Earthquake
focal mechanisms (e.g. 11/01/78 and 10/26/84) along the
Darvaz fault are consistent with geological evidence that
the Darvaz fault is a highly active, sinistral strike-slip fault
with a slip rate of around 10^15 mm a{1 (Burtman & Molnar
1993). The Vakhsh thrust, to the north of the Peter I range,
and the Trans-Alai thrust, to the northeast of the Pamirs,
display active N^S thrust faulting (mechanisms 12/18/77 and
08/12/88), although there is also evidence for right-lateral
strike-slip faulting within the region, as suggested by mechß 1998 RAS, GJI 134, 573^595
591
anisms 08/25/83 and 04/17/90. North of the Pamirs there is
active NNW^SSE thrusting at the northern margin of the
South Tien Shan (e.g. 10/13/85 and 05/06/82). There is also
evidence for NW^SE dextral strike-slip faulting along the
Talaso^Ferghana fault (mechanism 02/13/82). The NW^SE
trend of seismicity running from 770E, 310N to 730E, 350N
represents shallow northeastward subduction of the Indian
plate beneath the Himalayas. West of 730E, to the west of
the Hazara syntaxis, the earthquake mechanisms suggest
northwestward subduction of the Indian plate.
Nearly due north of the Hazara syntaxis, between 36.80N
and 37.50N, there is a N^S-trending cluster of events which
indicate E^W extension (e.g. mechanism 03/05/90). This
cluster of events relocates within the 40^60 km depth range
and not at the surface. However, further north, at 38.50N, there
is another mechanism displaying E^W extension. Geological
evidence for E^W extension within this region of the central
Pamirs is seen in N^S-trending extensional faults around a lake
near Ozera Kara Kul (73.50E, 390N) (Searle 1996a). Burtman
& Molnar (1993) suggested that this E^W extension might
be indicative of a cessation of crustal thickening within the
Pamirs, as did England & Houseman (1989) for Tibet. A
notable feature of the seismicity in the 0^75 km depth range is
the lack of signi¢cant seismicity along the NW^SE-trending
dextral Karakoram fault. This fault is clearly seen on satellite
images of the region, is generally regarded as one of the most
signi¢cant tectonic features in the region, and is believed to
have a high present-day slip rate of as much as 32 mm a{1
(Avouac & Tapponnier 1993).
2. 75^100 km depth range (Fig. 3)
The seismicity in this depth range is restricted to a much
smaller area than the shallower seismicity, and there seems
to be little correlation between the 0^75 and the 75^100 km
depth events. The relocated seismicity in the 75^100 km depth
range can be considered as three clusters, marked a, b and c.
Cluster a de¢nes an E^W trend (not seen in the ISC seismicity).
The three CMT mechanisms within cluster a (04/27/85,
04/17/79 and 05/17/90) have horizontal T-axes, and for
the ¢rst two events parallel the trend of this group of
events. (This behaviour of the T-axes also will be seen in the
earthquakes in the 100^125 km depth range in this region.)
Cluster b shows a slight elongation in the NNE^SSW direction
relative to the width of the relocated seismicity zone, and CMT
mechanisms display subvertical T-axes and roughly horizontal
N^S-trending P-axes. Cluster c has an arcuate trend, varying
from E^W at its western end through to NE^SW at its
northeastern end. The seismicity south of 360N is generally
shallower than that of cluster c. Cluster c contains CMT
mechanisms with predominantly subvertical T-axes, and
B-axes which closely parallel the trend of the cluster.
3. 100^125 km depth range (Fig. 4)
The earthquakes form a narrow S-shaped band of seismicity
approximately 700 km in length and no wider than 40 km.
Gaps in seismicity exist along the seismic zone which may
suggest that this S-shaped seismicity does not represent a
single feature. Two of the largest gaps in seismicity along this
S feature occur where the trend of the S changes most
592
G. Pegler and S. Das
sharply, and are marked d and e. Gap d was also identi¢ed
by Chatelain et al. (1980) and Roecker et al. (1980). It is
approximately 40 km long and marks the boundary between
what is often referred to in the literature as the Hindu Kush
(to the southwest) and the Pamir (to the northeast) seismic
zones. Clearly, the shallower clusters a and c described for
the 75^100 km depth range are located along the up-dip continuation of the S-shaped seismic zone. Cluster b occurs at the
southern end of the Pamir seismic zone and straddles a portion
of the seismic gap d. As discussed in the paper, we do not
consider cluster b to be an up-dip continuation of the S-shaped
seismic zone. The CMT mechanisms for earthquakes north
of 370N show a variation of types and were also discussed by
Fan et al. (1994). Two events (03/07/82 and 03/26/88) have
horizontal T-axes which parallel the strike of the S-shaped
seismic zone. This is similar to the behaviour of the T-axes
in this region at the 75^100 km depth range. Some mechanisms between 370N and 380N have B-axes which parallel
the NE^SW strike of the S-shaped zone and T-axes which
plunge to the southeast, roughly parallel to the southeast
dip of the S-shaped seismic zone in this region. However,
all CMT mechanisms within the Pamir seismic zone between
100 and 125 km depth and west of 740E have P-axes which
are perpendicular to the strike of the S-shaped zone and
plunge to the northeast. South of 370N, within the Hindu
Kush seismic zone, all CMT mechanisms have approximately
vertical T-axes, and B-axes which generally parallel the strike
of the seismic zone, that is the classical pattern for subduction.
4. 125^150 km depth range (Fig. 5)
This depth range, though less active than the 100^125 km
range, shows a continuation of the S-shaped seismic zone that
was clearly seen in Fig. 4. The width of this S-shaped seismic
zone is about 20 km for the well-relocated events at this depth.
Again, gaps of up to 60 km in length occur along the seismic
zone but the apparent continuity of the trend of the
seismic zone between the gaps is remarkable. The seismic gap,
marked d in Fig. 4, is wider at the 125^150 km depth range
than at the 100^125 km depth range, but an extrapolation
of the trend of the Pamir seismic zone around 370N, across
seismic gap d, would link it up with the ENE^WSW-trending,
slightly arcuate Hindu Kush seismic zone. Only three events
from the 125^150 km depth range have CMT solutions, all are
within the Hindu Kush seismic zone and two have near-vertical
T-axes.
5. 150^175 km depth range (Fig. 6)
The seismicity is sparse and the earthquakes are small. A
large number of small events, assigned mb ~0.0, are reported
by the ISC for the 150^175 km depth range, most of which
had too few reported phase arrivals to be relocated. The
well-constrained relocated events occur in four clusters,
two beneath the Pamirs and two beneath the Hindu Kush.
Each cluster is elongated along the S-shaped arc, suggesting
continuity of this feature, although the combined lengths of the
gaps in this depth range are almost of the same size as the arc
lengths with seismicity. The curvature of the S-shaped feature
has decreased from that seen for the 100^125 km range. The
single CMT in the 150^175 km depth range is within the Hindu
Kush seismic zone and again it has a vertical T-axis.
6. 175^200 km depth range (Fig. 7)
The earthquakes are larger than in the 150^175 km depth
range and have two dense clusters, one in the central and one
in the eastern Hindu Kush seismic zone, as well as a narrow
forked seismic zone in the western Hindu Kush and a narrow
NE^SW-trending Pamir seismic zone. All six available CMT
mechanisms in the central Hindu Kush cluster have steeply
southeast-plunging T-axes.
7.
200^225 km depth range (Fig. 8)
This depth range is characterized by intense seismic activity
along the arcuate Hindu Kush seismic zone with many large
events, with no activity (in the study period) along the Pamir
seismic zone. Almost all available CMT mechanisms within
the Hindu Kush seismic zone display near-vertical T-axes,
although most of the T-axes plunge towards the northeast,
rather than the southeast as for the 175^200 km depth range.
The CMT mechanisms within the western and central Hindu
Kush seismic zone have B-axes which parallel the zone;
however, within the eastern part of the seismic zone a clear
pattern is not obvious. A cluster of events occurs to the south of
the main arcuate Hindu Kush seismic zone at its eastern end.
This cluster of events, located near (36.40N, 71.20E), is also
seen in the next depth range.
8.
225^250 km depth section (Fig. 9)
The cluster at (36.40N, 71.20E) is elongated NE^SW in this
depth range into a second zone of seismicity, paralleling
the main arcuate Hindu Kush seismic zone, some 30 km to
the southeast. The horizontal projections of the 90 per cent
con¢dence ellipses for the better-constrained events show that
the cluster of events to the southeast of the main Hindu Kush
seismic zone are truly separated from the main seismic zone.
Four of the six CMT mechanisms within the 225^250 km
depth range in the eastern Hindu Kush seismic zone have
E^W-oriented P-axes, and one event has a normal mechanism.
9.
250^300 km depth range (not shown; see Pegler 1995)
At this depth, seismicity exists along the Hindu Kush seismic
zone which extends eastwards of that within the 225^250 km
depth range. The earthquakes were too small to have CMT
solutions. Other deeper sections are also not shown, although
one well-constrained event is located at around 380 km depth
beneath the eastern end of the Hindu Kush seismic zone
(Katok 1988) and is seen in the section R^R' shown later.
Seismicity cross-sections
The locations of the vertical cross-sections are shown in
Fig. 10.
A.
Section N^N' (Fig. 11)
The relocated earthquakes de¢ne a north-dipping seismic
zone at 100^180 km depth beneath the Hindu Kush, which
may extend to the surface approximately 300 km to the south
of the centre of the section in the vicinity of the westward
continuation of the Main Mantle and Main Karakoram
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
thrusts (MMT & MKT, Fig. 1). The north-dipping zone
appears to be about 30^40 km wide and the best-constrained
relocated events suggest a steepening of the seismic zone in the
100^180 km depth range relative to its dip between the surface
and 100 km depth. The events at the northern margin of the
South Tien Shan represent thrusting of the Tien Shan over
the Ferghana basin. A south-dipping feature in the 10^40 km
depth range at the northern margin of the Hindu Kush and
focal mechanisms (07/03/84 and 04/14/80) suggest that the
Hindu Kush is being thrust over the southern margin of
the Tadjik basin. The shallow, di¡use seismicity between the
Hindu Kush and Tien Shan (including the 07/05/90 event)
indicates thrusting within the Tadjik basin.
B.
Section O^O' (Fig. 12)
This section shows features similar to section N^N'. The
north-dipping seismic zone beneath the Hindu Kush displays
more seismicity in here than in N^N' and appears to have
a convex-up curve, dipping at around 500 in the 70^140 km
depth range and 80^900 in the 170^300 km depth range. The
seismic zone may extrapolate to a cluster of events at 30 km
depth, 270 km south of the centre of the section. We see a
similar pattern in the vicinity of the westward continuation of
the MMT and MKT, but if so there is a gap of 50^60 km with
no well-constrained events. Thrusting of the Tien Shan over the
southern margin of the Ferghana basin can be inferred from
event 10/13/85. Another feature of section O^O' is di¡use
shallow seismicity under the Tadjik basin.
C.
Section P^P' (Fig. 13)
This section also shows the north-dipping Hindu Kush seismic
zone in the 90^280 km depth range, with the well-constrained
relocated hypocentres de¢ning a zone no more than 40 km
thick. However, seismicity is not continuous along the zone
but is concentrated in two areas, between 80 and 150 km and
180 and 240 km depth, which dip at 500 north and vertically,
respectively. The seismicity in the 180^240 km depth range has
been referred to as the Hindu Kush nest of activity (Pavlis &
Hamburger 1991). At approximately 80 km depth, 100 km
south of the centre of the section, the width of the seismic
zone would appear to increase, although, as will become more
evident in section Q^Q', this may represent an imbrication
of the north-dipping seismic zone. Sparse seismicity in the
30^80 km depth range beneath the south-central Hindu Kush
may indicate the up-dip continuation of the Hindu Kush
seismic zone. Section P^P' also incorporates the northwestern
margin of the Pamirs beneath which there is shallow seismicity.
D.
Section Q^Q' (Fig. 14)
The north-dipping Hindu Kush seismic zone is still seen. The
part of it in the 60^100 km depth range is more clearly imaged
in this than any other section and de¢nes a narrow seismic
zone, less than 30 km wide, which dips at approximately 200 to
the north. This zone may extrapolate to a cluster of earthquakes at 40 km depth, 250 km to the south of the centre of
the section. Beneath the highest part of the Hindu Kush range
(100 km to the south of the centre of the section) there
appears to be an o¡set (north-up, south-down) in the Hindu
Kush seismic zone as it bends sharply at around 100 km
ß 1998 RAS, GJI 134, 573^595
593
depth. As with section P^P', there are few earthquakes within
the 150^180 km depth range and a region of high activity
between 180 and 270 km depth. The cluster of events in the
180^270 km depth range shows an inverted V-shaped feature.
The northern prong of the inverted V displays the higher level
of seismicity and dips at approximately 800 to the north. As
was seen in the 225^250 km depth range (Fig. 9), the northern
prong of the inverted V is continuous with the seismicity along
strike westwards around the Hindu Kush seismic zone; the
other prong of the inverted V is caused by the cluster of events
to the southeast of the main Hindu Kush seismic zone. Another
notable feature of this section is in the 70^100 km depth range,
from 80 km to the west of the centre of the section to 20 km
east of the centre of the section, where a 100 km long, 30 km
wide seismic zone, dipping at 100^150 north, links up with the
north-dipping Hindu Kush seismic zone. This seismic zone
represents cluster b of Fig. 3, and lies above and to the north of
the Hindu Kush seismic zone, with focal mechanisms which
show N^S shortening (09/15/86 and 02/05/90).
E.
Section R^R' (Fig. 15)
This is the easternmost N^S section in which the Hindu Kush
seismic zone is seen. (To the east of 71.50E, only the Pamir
seismic zone exists, as we shall see later.) The most notable
features of section R^R' are the two clusters of events in
the 75^110 km depth range. The clusters appear quite broad
(50 km) because section R^R' is slightly oblique to both
the Hindu Kush and the Pamir seismic zones. The cluster
of events at 230 km depth represents seismicity along the
southern prong of the inverted V within the Hindu Kush
seismic zone. The well-constrained events deeper than 150 km
depth show the Hindu Kush seismic zone to be at least vertical,
and the deepest events, a cluster at 280^290 km depth and a
single well-constrained event at 380 km depth, suggest that the
Hindu Kush seismic zone is overturned beneath about 240 km
depth.
F.
Section S^S' (Fig. 16)
Only one event is seen at around 80 km depth, along
strike from where the Hindu Kush seismic zone exists further
west. A cluster of events at the southern end of this section
occurs at 20^50 km depth beneath the MCT and MMT. The
dominant feature of this section is the di¡use seismicity in
the 100^240 km depth range, which represents seismicity of the
Pamir seismic zone. The intermediate-depth seismicity appears
di¡use due to the obliqueness of this section to the Pamir
seismic zone.
G.
Section T^T' (Fig. 17)
The Pamir seismic zone is clearly seen in this section as a
very narrow (20 km) seismic zone dipping to the southeast
at approximately 600 between depths of 100 and 160 km. ISC
epicentres (Pegler 1995) and the poorly constrained relocated
events suggest that the Pamir seismic zone may extend to
greater than 200 km depth. The cluster of events at 0^80 km
depth, about 160 km north of the centre of the section,
represents activity beneath the northern margin of the Pamirs.
It is di¤cult to see any clear trends within this shallow
594
G. Pegler and S. Das
seismicity, although a near-vertical feature seems to be fairly
well imaged. Further to the north, the shallow activity
represents thrusting at the northern margin of the Tien Shan.
The cluster of events at around 30^50 km depth, 180^250 km
to the south of the centre of this section, may represent
thrusting along a gently northward-dipping plane, which
may extrapolate to the surface around the Nanga Parbat
syntaxis.
H.
Section U^U' (Fig. 18)
The well-constrained relocated earthquakes in this section
also show the Pamir seismic zone between 100 and 140 km
depths and the less well-constrained events down to 250 km.
The zone has a southeasterly dip of approximately 400 in
the 100^140 km depth range. Shallow seismicity beneath the
northern Pamirs is associated with the Darvaz^Karakul and
Vakhsh fault systems. As with section T^T', it is di¤cult to see
any obvious trends within this shallow seismic zone, although
a near-vertical trend associated with the large 11/01/78
strike-slip earthquake is discernible.
I.
Section V^V' (Fig. 19)
The intermediate-depth seismicity of the Pamir seismic zone in
this section shows two well-constrained clusters of events. The
shallower cluster at 100^120 km depth dips southeast at
approximately 300 and contains two events with CMT
mechanisms (12/04/92 and 08/20/91) which have T-axes
paralleling the dip of the cluster. The deeper cluster of events
at 170^220 km depth suggests that the dip steepens to
approximately 700 between 120 and 220 km. The `two-slab'
model would require linking up these earthquakes between 100
and 220 km depth to the seismicity of the shallow cluster
(which is situated more than 130 km horizontal distance to
the northwest, and includes the events 01/09/88, 10/26/87 and
11/03/90), even though there is no seismicity between 70 and
100 km depth to indicate such a link. The Darvaz^Karakul
and Vakhsh fault systems again show activity in the 0^30 km
depth range and the 01/31/77 event indicates thrusting along
the northern margin of the South Tien Shan. The cluster of
events between 40 and 70 km depth 80 km to the southeast of
the centre of this section are the E^W extensional events near
730E, 370N (Fig. 2). The north-dipping trend of seismicity in
the 20^60 km depth range, 200 km to the south of the centre
of the section, was partly imaged in section T^T'. However, this
section shows more clearly the northward dip of this feature,
which may extrapolate to the surface in the vicinity of the
MMT.
J.
Section W^W' (Fig. 20)
The geometry of the Pamir seismic zone in this section
is similar to that in section V^V'; however, the level of
seismicity is higher and there is no gap in seismicity within the
120^170 km depth range. As with all the other sections taken
across the Pamir seismic zone, the seismicity at the top of the
southeast-dipping, intermediate-depth seismic zone terminates
abruptly at around 90 km depth; it is seen most clearly in this
section. Shallow seismicity is again seen at the northern margin
of the Pamirs, although in this section it appears more like a
south-dipping wedge than in sections T^T' and U^U'. Again,
the two-slab model would require the slab to continue upwards
and northwestwards to connect with the shallow cluster to
the right of the centre of the section, though practically no
seismicity exists in the region in between.
K.
Section X^X' (Fig. 21)
This section is taken through the eastern end of the Pamir
seismic zone. This is the least seismically active part of the
intermediate-depth Pamir seismic zone. Indeed, for sections
taken east of 740E it is necessary to consider data from a
section at least 100 km wide in order to see any obvious
geometry within the seismic zone. The ISC data for this section
(not shown; see Pegler 1995) show a thin seismic zone dipping
at approximately 700S between 100 and 150 km depth. The
relocated seismicity de¢nes a more vertical trend and activity
over a slightly smaller depth range. The shallow seismicity
occurs predominantly beneath the Tarim basin but there is no
clear suggestion of any trends within the seismicity.
L.
Section Y^Y' (Fig. 22)
This section is approximately parallel to the trend of much
of the Pamir seismic zone and gives another view on the
seismicity. The top of the intermediate-depth seismicity of
the Pamir seismic zone appears to have an undulating upper
surface, which can be divided into three segments. The ¢rst
runs from the Hindu Kush seismic zone (120 km from Y),
where the top of the seismic zone is at approximately 75 km
depth, along to the centre of section Y^Y', where its top is
at about 100 km. The second segment continues for 100 km
further northeast, with a convex upper surface, and the third
segment continues until 220 km east of the centre of Y^Y',
where the top of the Pamir seismic zone is at a depth of about
90 km.
Sections Z1^Z8
A set of thinner, more arc-perpendicular sections is presented
to examine the region where the Hindu Kush and Pamir
seismic zones `meet'. Fig. 23 shows the location of this set of
sections. Sections Z1 to Z8 are shown in Figs 24 and 25.
Sections Z1 to Z5 show the north-dipping Hindu Kush
seismic zone. However, this zone is only imaged shallower than
100 km in sections Z1 and Z2. The eastward disappearance
of the up-dip continuation of the Hindu Kush seismic zone
coincides with the ¢rst appearance of the cluster of events to
the north of the Hindu Kush seismic zone (cluster b of Fig. 3).
The eastward steepening of the Hindu Kush seismic zone in the
100^300 km depth range is also seen between sections Z1 and
Z5, with the overturning of the seismic zone in Z4 and Z5.
Sections Z1 to Z4 also allow a closer analysis of the inverted
V-shaped structure seen in section Q^Q'. Within section Z1,
between 180 and 220 km depth, there is a clear 850 dip to the
south in a zone of seismicity which cuts across the northdipping seismic zone. This feature was noted by Billington
et al. (1977), who identi¢ed the feature as a fault plane within
the western part of the Hindu Kush seismic zone (Fig. 6. of
Billington et al. 1977). However, sections Z2, Z3 and Z4
ß 1998 RAS, GJI 134, 573^595
Pamir^Hindu Kush seismic zone
suggest that this feature has developed further eastwards
within the Hindu Kush seismic zone than inferred by Billington
et al. (1977).
Seismicity beneath the Karakoram
The intermediate-depth seismicity beneath the Pamirs
terminates eastwards at around 75.50E. However, there are
some intermediate-depth events further to the southeast,
beneath the Karakoram. These events have been interpreted
as evidence for the deep subduction of the Asian lithosphere
beneath the Karakoram (Fan et al. 1994). Fig. 26 shows
ß 1998 RAS, GJI 134, 573^595
595
the seismicity of the Karakoram region. Section KA^KA'
is a section taken through this region oriented roughly
perpendicular to the strike of the Karakoram fault, the MKT
and the Himalayan thrust front to the south. Clearly, the
seismicity de¢nes a triangular wedge which extends to a depth
of about 100 km. The north-dipping southwestern boundary
of the wedge intersects the surface along the MKT, and the
northern boundary of the wedge intersects the surface at the
southern margin of the Tarim basin. The CMT mechanism
shown in section KA^KA' is that of the 02/13/80 earthquake and is interpreted as thrusting along the MKT at 77 km
depth.
Figure 1. Map showing the Pamir^Hindu Kush region. Topography is contoured at 1000 m intervals (0^1000 m: green; 1000^2000 m: yellow; etc.)
Faults are marked in black. VT~Vakhsh Thrust; DKF~Darvaz^Karakul Fault; KaF~Karakoram Fault; AF~ Andarab Fault; HF~Herat Fault;
PF~Panjer Fault; CF~Chaman Fault; ISZ~Indus Suture Zone; MBT~Main Boundary Thrust; MKT~Main Karakoram Thrust; MMT~Main
Mantle Thrust; NP~Nanga Parbat; H~Haramosh; HS~Hazara Syntaxis; ATF~ Altyn Tagh Fault; AKF~Atushi^Keping Fault; TFF~Talas
Ferghana Fault. Coloured contours represent the intermediate-depth seismicity of the Pamir^Hindu Kush seismic zone (red: 100 km; orange:
125 km; yellow: 150 km; green: 175 km; turquoise: 200 km; blue: 225 km; purple~: 250 km). The black arrow shows the direction of the plate motion
of India relative to Eurasia (DeMets et al. 1990).
ß 1998 RAS, GJI 134, 573^595
ß 1998 RAS, GJI 134, 573^595
Figure 10. Map showing location of seismicity sections. Contours represent the intermediate-depth seismicity of the Pamir^Hindu Kush seismic
zone (red: 100 km; orange: 125 km; yellow: 150 km; green: 175 km; turquoise: 200 km; blue: 225 km; purple: 250 km). Faults are marked in black
(Searle 1996b).
Figure 2. Relocated seismicity depth range 0^75 km for the period 01/01/1964^12/31/1992, together with selected CMT mechanisms. Solid dots
represent epicentres with 90 per cent con¢dence limits ¦30 km. Hollow dots represent the remaining relocated events which have greater hypocentral
uncertainties and are plotted at half the diameter of the solid dots for events with equivalent mb . The size of each dot is related to the earthquake
magnitude, the size key being shown in Fig. 5. (Note that the smallest dots may appear solid but are actually hollow circles.) Few ISC events with
indeterminate depths are successfully relocated. Faults are marked in black. Contours representing the intermediate-depth seismicity of the Pamir^
Hindu Kush seismic zone are the same colours as in Fig. 1. CMT mechanisms are plotted as lower-hemisphere projections with compressional
quadrants in red. The month, day and year for the CMT solutions are shown next to them. To maintain clarity in the ¢gure, only su¤cient CMT
solutions to indicate the type of faulting in the region are shown. For example, in regions of high activity with many CMT mechanisms for earthquake
clusters, only a few are shown.
ß 1998 RAS, GJI 134, 573^595
Figure 26. Seismicity across the Karakoram region.The map view (upper) shows all relocated seismicity covering the period 01/01/1964^12/31/1992.
The location of section KA^KA' (lower) is also shown. Contours represent the intermediate-depth seismicity of the Pamir seismic zone. CMT
mechanisms for the 07/29/77 and 02/13/80 events discussed in the text are shown as lower- and back-hemisphere projections in map view and section,
respectively. Fault are shown in black. MKT~Main Karakoram Thrust. Units etc. are the same as in earlier map-view and cross-section ¢gures.
ß 1998 RAS, GJI 134, 573^595